A combined biochemical and anatomical study is underway to identify neurotransmitters of retinal neurons and in neurons of visual pathways, to examine the development of neurotransmitter systems in retina, and to determine the role of chemical neurotransmitters in the processing of visual information. Two systems are being investigated: (1) the simple visual system of the horseshoe crab Limulus polyphemus and (2) developing mammalian retinal neurons. (1) Studies of neurotransmitters in the Limulus visual system revealed that retinal efferent fibers (fibers which project from cell bodies in the brain to the retina) synthesize, store, and release the neuromodulator octopamine. Conjugates of octopamine were also released from the same fibers when the intensity of stimulation was increased. Limulus photoreceptor cells were shown to respond to octopamine by increasing intracellular cAMP. Analyses of anatomical projections of efferent fibers to Limulus retinas showed that the fibers innervate specifically the photosensitive membrane of photoreceptor cells. These studies provide the first clear identification of a neurotransmitter in retinal efferent fibers and open the way for studies of direct neuromodulation of such basic functions of photoreceptor cells as phototransduction and the turnover of photoreceptive membrane. (2) The control of development of synaptic function of mammalian retinal neurons is being studied using monolayer cultures prepared from embryonic rat retinas. The influence of hormones, cAMP, and activity on the synthesis and release of neurotransmitters is being investigated. Steroid hormones and cAMP were found to accelerate specifically the release of acetylcholine (ACh) from retinal neurons without influencing acetylcholine synthesis. Depolarization of the cells in culture profoundly increased the GABA synthetic activity measured. These experiments indicate that a number of factors can profoundly influence the development of retinal neurons.